1. Field of the Invention
The present invention relates to a driving unit of a vibration-type actuator that excites a vibrating body having at least an electro-mechanical energy conversion element by supplying an alternating current (AC) signal (AC voltage) to the electro-mechanical energy conversion element to drive a movable body that is in contact with the vibrating body.
2. Description of the Related Art
The driving speed of vibration-type actuators, such as ultrasonic motors, changes depending on the amplitude of vibration excited on a vibrating body. Therefore, the driving speed is controlled using the amplitude or frequency of an AC signal supplied to an electro-mechanical energy conversion element, such as a piezoelectric element, provided at the vibrating body. It is common to employ a control method using a frequency to reduce fluctuations in resonance frequency due to temperature or the influence of individual differences. In terms of a frequency generating method, there are a digital method using an analog oscillator or quartz oscillator and a programmable frequency divider and a semidigital method in combination with a digital-to-analog converter.
However, such a digital method has a problem in achieving high frequency resolution.
Japanese Patent Laid-Open No. 06-339289 proposes a method for falsely improving the frequency resolution by periodically changing the frequency dividing rate and using an average frequency dividing rate. Furthermore, it presents a method of increasing the periodic change speed to shift the vibration from an audible range when the periodic change causes vibration in the audible range.
The Publication of Japanese Patent No. 3504130 proposes a method by which an actuator that moves by a predetermined distance every period of a periodic signal using a pulse motor or a piezoelectric element changes its period command little by little and uses the average period. As a method for changing the period command little by little, uniform random numbers are added to the period command of the periodic signal to prevent the period command from periodically changing, thereby preventing generation of a specific audible sound.
However, the methods disclosed in the Publication of Japanese Patent No. 3504130 and Japanese Patent Laid-Open No. 06-339289 change the frequency of AC voltage. Therefore, if frequency components contained in the change are in the vicinity of the natural frequency of the vibrating body or in the vicinity of the natural frequency of a mechanism driven by the actuator, the problems of audible sound or degradation in speed control function may occur. Furthermore, there is also the problem of promoting abrasion due to excitation of unnecessary vibration to degrade the durability.